1. Field of the Invention
This invention relates to selecting molecules from air or other gaseous or liquid matter based on the speed or direction of those molecules and generating bulk flows from the selected molecules.
2. Description of the Related Art
Molecules in air (and other gaseous matter) are in constant motion, continuously colliding with each other. This molecular motion is constantly occurring, even if the bulk velocity of the air is zero.
The speed of the molecules between collisions is the thermal velocity for the air. The average distance between collisions is the mean free path distance. The overall bulk velocity of the air is the air's transport velocity. Theoretically, the maximum transport velocity that can be imparted to an airflow is the thermal velocity of the underlying air molecules.
Several conventional apparatuses exist for generating a flow of air. Examples include fans and turbomolecular pumps.
Fans force air to flow in bulk with rotating fan blades. Even highly efficient fans cannot achieve very high transport velocities compared to the underlying molecular motion of the air. In particular, even good fans can only achieve transport velocities that are on the order of a hundredth to a thousandth of the thermal velocity of the air molecules.
Because high-velocity (i.e., comparable to thermal velocity) bulk air flow cannot be achieved with conventional fans, larger fans must be used to move significant amounts of air. As a result, fan size often becomes a limiting design factor for anything that requires airflow, cooling, or the like.
Another device for moving air (and other gaseous matter) is the turbomolecular pump. Turbomolecular pumps can be used as absorbers or consumers of air molecules. These pumps typically are used to draw molecules from a high vacuum environment in order to create an even “higher” vacuum.
Turbomolecular pumps use rotating turbine blades to select molecules from air. Molecules that randomly cross the tops of the blades are captured and whisked away.
In order for existing turbomolecular pumps to operate, collisions between air molecules must be avoided. If such collisions occur, the molecules can bounce away from the blades before they can be captured, defeating the operation of the pump.
Typical existing turbomolecular pumps use macroscopic turbine blades rotating at extremely high speeds, for example 75,000 RPM. These high speeds are used so that molecules that cross the path of the rotor blades do not have time to collide with other molecules before being whisked away.
Collisions are also prevented by ensuring that the mean free path distance for the molecules is not too small compared to the container or feed tube for the pump. The ratio between container or feed tube length and mean free path distance is the Knudsen number.
Typical existing turbomolecular pumps only operate effectively if the Knudsen number is no greater than approximately 10. This Knudsen number can only be achieved in a high vacuum, and then with only relatively small containers or feed tubes. Obviously, a significant air flow cannot be generated by pumping from a high vacuum through a small container or feed tube. As a consequence, existing turbomolecular pumps do not generate significant air flow.
All of these problems also exist when generating a flow from any other gas or gas mixture besides air.
Generation of flows from gasses is of interest because such flows are ubiquitous in modern technology. For example, heating and cooling applications generally utilize some type of bulk flow in their operation. Examples of these applications include cooling units for computers, radiators for cars, air conditioners, refrigerators, heaters, industrial cooling units for large machinery, and innumerable other devices.
Conventional techniques for exchanging heat with air involve forced convection. In forced convention, air is forced to flow over or through some heating or cooling element. For example, air can be blown over or through a heated or cooled substrate, duct or grille. The purpose of these arrangements can be to heat or cool either the air or the substrate, duct or grille.
In all of these arrangements, a boundary layer forms over the surface of the substrate, duct or grille. In particular, air molecules in contact with the surface tend to “stick” to the surface. These air molecules in turn impede the motion of adjacent air molecules in the air flow, which in turn impede other air molecules. Thus, a region of slow-moving air molecules forms over the surface. This region is known as a velocity boundary layer.
The velocity boundary layer limits the number of air molecules that come into contact with the surface. Actual heat transfer only occurs at this surface. As a result, once heat is transferred to or from the molecules in the boundary layer, further transfer of heat is largely blocked. More heat can only be transferred once the molecules in the boundary layer are dragged away from the surface by the viscosity of the air, which is an intrinsically inefficient process. Molecular collisions also can drive the molecules away from the surface, but this is an even more inefficient process. As a result, the velocity boundary layer is accompanied by a thermal boundary layer.
The thermal boundary layer greatly impedes the transfer of heat between the forced air and the substrate, duct or grille. In addition, the forcing elements (e.g., fans) for conventional heat transfer devices must be powerful enough to overcome the viscosity of the air. Otherwise, little heat transfer will occur. Because of these factors, heating and cooling units tend to be fairly large devices with large footprints. These large footprints are the limiting design factors in many modern devices.
One alternative technique that has been explored with little success is heating or cooling the blades of fans that force (i.e., blow) air. However, in this approach, a thermal boundary layer forms on a fan's blades. As a result, this approach is no more efficient than forcing air over or through a substrate, duct or grille. All of these problems also exist when transferring heat to or from any other gas or gas mixture besides air.